This invention relates to a metal cord for reinforcing a rubber article having excellent corrosion resistance and fatigue resistance and to a composite material comprising rubber and the metal cord embedded in the rubber as a reinforcing material, such as a vehicle tire, a conveyor belt and a high-pressure hose.
A metal cord for reinforcing a rubber article is made of high carbon steel wires (JIS G 3502 piano wire rods) having their surfaces placed with brass, copper or zinc in order to impart thereto an adhesive property with rubber. Such wires are drawn to a diameter of 0.1 to 0.5 mm and are then twisted in a single layer or in a plurality of layers. They are widely used as reinforcing materials for vehicle tires, conveyor belts and high-pressure hoses. For example, of a radial tire for a passenger car, metal cords having a 1.times.4 or 1.times.5 single-layer twisting construction which are formed by twisting four or five metal filaments together are used as a reinforcement of the belt portion.
The quality characteristics required for a metal cord for reinforcing rubber include adhesion with rubber, resistance to corrosion, resistance to fatigue and various mechanical properties (such as breaking load, rigidity, etc.). Firstly, we shall discuss the resistance to corrosion. If part of the metal cord embedded in rubber were not covered with rubber, or if the tire were to ride over a stone or a nail while travelling on a road and the damage in the tire caused by the stone or nail were to reach the metal cord, water could infiltrate through the damaged part into the cord and cause a premature corrosion of the cord, thus lowering its breaking load and fatigue resistance. Also, the adhesion strength between the cord and rubber will decrease, causing peeling and thus separation. The quality of the tire would drop markedly.
The widely used 1.times.5 or 1.times.4 closed cords (C.C.) have sections as shown in FIGS. 4A, 4B. In the center of these metal cords, there exists a rubber-free hollow space because rubber penetration into the cord is insufficient. A composite material comprising rubber and a metal cord having such a hollow space has a short life, because water can infiltrate into the hollow space, thus causing such problems as corrosion of the metal cord and separation between the metal cord and rubber.
Trials have been made to eliminate such rubber-free portions to improve the metal cords. For example, Japanese Unexamined Patent Publication 55-90962 discloses an open cord as shown in FIG. 5 in which the metal filaments 2 are twisted together in such a way as to form gaps between the adjacent metal filaments. Japanese Examined Patent Publication 58-31438 proposes as shown in FIG. 6 a parallel 2+2 cord having substantially no hollow space inside. Both cords are actually being used as reinforcing materials for tires.
Efforts are being made to reduce the emission of exhaust gas from automobiles in order to answer the growing concerns about environmental protection. Vigorous efforts are especially directed to the development of low fuel-consumption cars, because reductions in fuel consumption directly lead to reductions in the emission of exhaust gas. It is also required that vehicle tires to be lightweight in order to reduce the rolling resistance as well as the entire vehicle weight. For this purpose, trials have been made to reduce the diameter of the metal cord and to simplify the twisted configuration of the cord to reduce the thickness of the rubber covering.
For example, Unexamined Japanese Patent Publications 62-117893 and 62-234921 disclose very simple 1.times.2 cords in which two metal filaments 2 are simply twisted together as shown in FIGS. 2(A)-2(C). This type of cord is free of spaces inside and thus their filaments are entirely covered with rubber aspect for the contact portions between the filaments. Resistance to corrosion is thus good. Unexamined Japanese Patent Publication 2-229286 teaches a 1.times.2 cord in which the degree of preshaping of the two filaments are set within such a range as to provide partial gaps between the metal filaments, thus improving the rubber covering ratio and thus the corrosion resistance.
The 1.times.2 cords disclosed in Unexamined Japanese Patent Publications 62-117893 and 62-234921 exhibit excellent corrosion resistance because they are completely covered with rubber and also serve to reduce the weight of vehicle tires because they are made up of a minimum number of filaments. But they have drawbacks. Namely, in order to use 1.times.2 cords in a passenger car tire in place of 1.times.4 or 1.times.5 cords, which have heretofore been widely used, filaments having a larger diameter had to be used in order to maintain the same tenacity and rigidity as those of the metal cords made of greater number of filaments. Also, as viewed in cross section as in FIGS. 2B and 2C, 1.times.2 cords have their filaments arranged either horizontally of vertically. Thus, they have irregular shapes in the longitudinal direction.
A cord made of metal filaments having a large diameter and having irregular shapes in the longitudinal direction exhibits inferior resistance to fatigue (bending fatigue and compressive fatigue). If it is used in a belt portion of a passenger car tire, compressive stress will act on the metal cord repeatedly due to buckling while the car is turning a corner at high speed. This cord exhibits poor resistance to compressive fatigue to such repeated buckling.